1. Field of the Invention
This invention relates generally to computer memory devices and relates more particularly to a memory device for reading data by measurement of the tunnel current.
2. Description of the Related Art
The scanning tunneling microscope (STM) proposed by Gerd Binnig and Heinrich Rohrer of IBM is a device for measuring, on an atomic scale, the surface contours of a sample by bringing a probe having a sharp tip of a radius of curvature of about 10 nm within some tens of angstroms of the sample surface across which the tip is scanned and by detecting variations in the tunnel current.
While the STM in its early stage was of a bulk structure including vibration isolation equipment, the advent of the micromechanical technology has decreased the device in size to minimum and removed problems associated with vibrations and thermal drifts, as achieved by a group of Stanford University people headed by Calvin F. Quate, who succeeded in forming on a silicon substrate by an IC process a cantilever type STM provided with a cantilever generally formed of piezoelectric material and which is capable of three dimensional scanning.
A STM memory which reads data by recording digital data bits by selectively forming perturbations in the surface of a substrate, measuring the tunnel current flowing between the probe and the surface of the substrate while permitting the probe to be scanned across the surface, and by detecting the presence of the perturbations, is disclosed in U.S. Pat. No. 4,575,822 to Calvin F. Quate, entitled "Method and Means for Date Storage using Tunnel Current Data Readout". The writing of surface perturbations by using a STM has been disclosed by J. Schneir, R. Sonnenfeld and others in a report entitled "Tunneling microscopy, lithography and surface diffusion on an easily prepared atomic flat gold surface". The report discloses a hole of a width of 5 nm and a mound of 10 nm in diameter. It also discloses that a continuous observation by a STM indicated that the mound had been spread out and had become flat after 21 hours.
The cantilever type STM mentioned above is applicable to this STM memory. In a cantilever type STM of a size of 5 .mu.m.times.200 .mu.m.times.1000 .mu.m in which a vaporized ZnO film is employed as a piezoelectric element, the ZnO film is elongated or contracted by 22 .ANG./V in a longitudinal direction, by 220 .ANG./V in a lateral direction and by 7700 .ANG./V in a direction perpendicular to the surface, so that when it is driven at a voltage of 5 V the range of horizontal scan is on the order of 110 .ANG..times.1100 .ANG.. Such cantilever has sufficient stroke when used in a STM for monitoring the sample surface.
When, however, the cantilever is used for a STM memory its stroke is not enough to constitute a mass memory. That is, when the recording area per bit of a STM memory is set to be 5 .ANG..times.5 .ANG., the amount of data that is recorded on the horizontal scan range 110 .ANG..times.1100 .ANG. is on the order of 5 K bits. The amount of data that is recorded is on the order of 20 K bits even if the recording area per bit is set at 2.5 .ANG..times.2.5 .ANG.. Even if one-hundred STM memories are formed on a single chip thanks to an IC process, available memory capacity is only about 2 M bits and is subject to limitations to establish a mass memory.